1. Field of the Invention
This invention relates to organic polymeric isocyanates and more particularly pertains to an improved process for purifying and reducing the acidity in methylene-bridged polyphenylpolyisocyanate mixtures.
2. Description of the Prior Art
Organic isocyanates are generally prepared by the phosgenation of corresponding amine compounds by one of the many well-known phosgenation processes. These phosgenation processes usually result in crude isocyanate products that contain residual acidic materials which adversely affect the reactivity of the isocyanate. The acidic material contaminants are generally those which respond as acids in standard analytical tests known and employed in the art. The acidic material contaminants are known to include hydrogen chloride and a variety of unknown by-product materials of which only some appear to be hydrogen chloride precursors. These materials also respond as acids in the aforementioned standard analytical tests.
There are procedures known for purifying organic isocyanates to reduce acidity levels and otherwise increase their reactivity rates. Generally, the most common practice in the industry is to subject crude isocyanate mixtures to one or more distillation steps, such as by passing a stream of the crude reaction mixture of the phosgenation reaction mentioned hereinabove, which contains the organic isocyanate, solvent and impurities, through one or more distillation columns whereby the solvent employed in the phosgenation reaction and acidic material contaminants are carried overhead, leaving the organic isocyanate as residue. For example, U.S. Pat. No. 3,264,336 discloses the employment of fractional distillation as a method for removing acid contaminants from organic isocyanates.
However, conventional distillation techniques have left much to be desired in regard to reducing the acidity levels of organic isocyanates. Apparently, it is difficult to separate many acidic material contaminants from the isocyanate material by conventional distillation.
There are several procedures described in the literature for improving the separation of acidic material and other contaminants from isocyanates by distillation. For example, a number of methods have been described which comprise treatment of crude organic isocyanates with metals, such as copper, silver, nickel, iron, zinc, cobalt, aluminum, bismuth, and the like, and then distilling the mixtures for separation. It is disclosed that the metallic compounds form materials or complexes with acidic material contaminants of the crude isocyanate which can be readily separated by distillation, thereby resulting in an isocyanate having reduced acidity. See U.S. Pat. No. 3,155,699, 3,264,336, 3,373,182 and 3,458,558.
In addition, U.S. Pat. No. 3,219,678 discloses a process for purifying organic isocyanates for the reduction of the hydrolyzable chloride content therein which includes the steps of subjecting a crude organic isocyanate mixture that has been previously degassed and subjected to distillation for solvent removal (employed in the aforementioned conventional phosgenation procedure) to a temperature considerably above those temperatures required and used in conventional degassing procedures for the cleavage of organic carbamyl chlorides formed in the phosgenation reaction into organic isocyanate and hydrogen chloride, for extended periods of time, and then distilling the mixture to separate the organic isocyanate. It is disclosed that the heating prior to distillation apparently removes hydrolyzable chloride contaminants or those responsible for hydrolyzable chloride content which are not removed by simple distillation of the organic isocyanate. However, the process disclosed in U.S. Pat. No. 3,219,678 leaves much to be desired from a commercial operation standpoint inasmuch as it would apparently require the utilization of a plurality of distillation columns and/or extensive tie-up of plant production equipment which necessarily reduce the economics of the process.
Furthermore, the above-mentioned procedures have especially left much to be desired for purifying and reducing the acidity levels of polymethylene polyphenylpolyisocyanate mixtures. Generally, polymethylene polyphenylpolyisocyanate mixtures are prepared by the well-known procedures of mixing and reacting phosgene, in the presence of a compatible solvent such as monochlorobenzene, with a corresponding methylene-bridged polyphenyl polyamine mixture prepared by the condensation reaction of formaldehyde and aniline or a related polyamine in the presence of a strong mineral acid or alumina-silica catalyst. Illustrative methods of the preparation of methylene-bridged polyphenyl polyamines and corresponding polymethylene polyphenylpolyisocyanates are described in U.S. Pat. Nos. 2,683,730; 2,950,263; 3,012,008; 3,344,162; and 3,362,979, to name a few. The primary disadvantage of employing the above-mentioned processes for purifying and reducing the acidity of polymethylene polyphenylpolyisocyanates is the fact that these materials are heat-sensitive. Exposure to high temperatures for extended time periods adversely affect the chemical and physical properties of polymethylene polyphenylpolyisocyanate mixtures, such as viscosity, isocyanate equivalent weight, weight percent free isocyanate, and the like. Furthermore, polymethylene polyphenylpolyisocyanates prepared by the aforementioned processes exist as mixtures of methylene diphenylisocyanate and higher functionality, higher molecular weight methylene-bridged polyphenylpolyisocyanates which have variable boiling points.
For these reasons, it has heretofore been a common practice in the industry to purify and reduce the acidity of polymethylene polyphenylpolyisocyanate mixtures by a distillation procedure employing a fractional distillation column having a reboiler consisting of a thin film evaporator means. More particularly, in accordance with conventional techniques, a crude polymethylene polyphenylpolyisocyanate mixture, from the phosgenation reaction containing solvent and impurities is initially degassed by rapidly heating the crude mixture to about 70.degree. to about 90.degree. C., under about 60 to about 90 mm. Hg absolute pressure, to remove unreacted phosgene and other highly volatile impurities, and then passed through the fractional distillation column and thin film evaporator means where the mixture is subjected to high temperature for only a few seconds to reduce acidity without significantly affecting the polymethylene polyphenylpolyisocyanate mixture physical and chemical characteristics. The solvent and impurities are taken overhead.
Although the distillation procedure described immediately hereinabove has been found to be effective in removing the solvent and high volatile impurities from the polymethylene polyphenylpolyisocyanate mixtures without adversely affecting the physical and chemical properties of the mixture, it has left much to be desired in regard to reducing the acidity of the isocyanate product.
Accordingly, it is the primary object of the present invention to provide an improved process for treating a polymethylene polyphenylpolyisocyanate mixture whereby the resulting isocyanate has a reduced acid level without adversely affecting the viscosity, isocyanate equivalent weight, isocyanate reactivity and like physical and chemical characteristics.
It is another object of the present invention to provide an improvement in the process for purifying and reducing the acidity level of a polymethylene polyphenylpolyisocyanate subjected to a distillation step for separation of the solvent employed in a conventional phosgenation reaction and other contaminants.
It is yet another object of the present invention to provide an improvement in the process for purifying and reducing the acidity of polymethylene polyphenylpolyisocyanate mixture subjected to distillation procedures which does not require the employment of an extensive series of distillation columns and related apparatus and/or extensive tie-up of plant equipment and related long time requirements.
Other objects and advantages of the present invention will become readily apparent to those having ordinary skill in the art from the following description of the invention along with the attached drawing.